The invention relates to DC-level temperature drift of an analog phase detector of a PLL (phase-locked loop). A conventional phase locked loop has a phase detector, a loop filter, and a feedback branch, which includes a voltage-controlled oscillator (VCO) coupled to the phase detector. While the phase difference between the incoming signals remains constant, the output level will drift as the temperature changes. In a PLL, detrimental temperature drift appears also in the loop filter and in the voltage-controlled oscillator VCO.
A PLL can be used for FM signal detection in FM-IF intermediate-frequency circuits.
In a phase-locked loop the thermal drifts of the different functions jointly offset the average frequency of the VCO to the degree that the loop cannot be caused to lock or that maintaining it locked becomes a problem.
This happens especially when the loop filter has been calibrated as a narrow-band filter and when the level of the incoming IF signal decreases from its nominal value. This situation appears, for example, when the receiver is listening to nothing but noise and the amplitude of the output signal of the limiter-amplifier remains deficient.
In theory it is possible to make each block of the PLL fully stable with respect to the temperature, but in reality there always appears temperature drift to some degree.
At its simplest the loop filter of a PLL comprises only resistors and capacitors. In that case there may in principle appear thermal drift in the loop only in the phase detector or the VCO.
The following compensation methods can be used with respect to temperature drifts:
A: If the temperature drifts of the different blocks are inverse to each other, the problem is eliminated. This results however, on the condition that the temperature coefficients change linearly or that the non-linearities are mirror images of one other. It is to be expected that at a great temperature change (range of change approximately 100.degree. C.) the coefficients do not completely annul one another. In this case the loop's remaining locked or becoming locked will constitute a problem at the extreme temperatures. This is also seen as a distortion of the detection result.
B. The necessary offset voltage can be summated into the loop filter to compensate for the drift errors caused by the temperature. The precision requirement of the offset voltage becomes a problem if the drift errors are in the same direction.
The most common method of compensation in PLL circuits has so far been the solutions according to point A. On the other hand, very few PLL-IF circuits are available commercially, and so there are not very many alternative examples. According to experience, at the extreme values of the operating temperature the remainder of the circuit locked and its locking constitute a problem at low signal levels.
It would therefore be desireable to find a circuit arrangement by means of which the temperature drift of a phase detector is compensated for in a simple and economical manner.